What Spelling Dictionaries Are Supported?
Nisus Writer supports many spelling dictionaries: all those supported by Apple/OSX. If you install a new spelling dictionary for OSX, just restart Nisus Writer and it should automatically appear in Nisus Writer's language preferences.

How Do I Install Additional Spelling Dictionaries?
As of Mac OS X 10.6 (and later) you can install Hunspell dictionary files directly and OS X will automatically make them available in Nisus Writer. Here's how:

There are many places that offer Hunspell dictionaries, but you might check the LibreOffice dictionary repository or OpenOffice extension repository. Some Hunspell dictionaries there can be downloaded directly, while most will download as OpenOffice Extension ".oxt" files. Once a ".oxt" file is downloaded, rename it to ".zip", extract it, and look inside the "dictionaries" folder. It will contain the necessary ".dic" and ".aff" Hunspell files. Keep those handy, as you'll need to move them later in the installation process.

3. In the Finder locate one of your Library folders (either your home or root Library).
Because Apple decided to start hiding Library folders from users, you may need to take some special steps to reveal the Library folder. The easiest is to use the Finder's menu Go > Go To Folder and then type "~/Library" without the quotation marks.

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Once you click the "Go" button you should see the Library folder revealed.

4. In the Library folder, find the "Spelling" subfolder. If no such folder exists you can create it.

5. From the Hunspell materials you downloaded and extracted in step 2, find both the ".dic" and ".aff" files and move them into the Spelling folder:

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That should complete the process, and if you relaunch Nisus Writer you should now see the installed dictionary in your Nisus Writer language preferences.

Note: a reminder that Nisus Writer supports any spelling dictionary available to OSX, so other alternatives are also possible, eg: Cocoa Aspell, which also has its own extendible set of free dictionaries.